A number of nonsurgical techniques for treatment of obstructed blood vessels have been investigated. Such techniques include balloon angioplasty wherein a passage is formed in the obstructed region with a dilatation balloon, laser balloon angioplasty wherein heat and pressure are simultaneously applied to enlarge a passage through a blood vessel, and the so-called "hot tip" technique wherein a high temperature heating element is pushed through plaque deposits to form a passage. Another widely investigated technique involves the use of radiant energy, typically laser energy, to remove obstructions such as plaque deposits by vaporization or ablation. The laser removal technique employs a catheter having one or more optical fibers extending through the catheter to its distal end. The catheter is advanced through a blood vessel to an obstructed site, and laser energy of a suitable wavelength and intensity to remove the deposit is directed through the optical fiber or fibers. Catheters for removal of obstructions with laser energy are disclosed in U.S. Pat. No. 4,817,701 issued Apr. 4, 1989 to Roth et al and U.S. Pat. No. 4,850,351 issued Jul. 25, 1989 to Herman et al.
A catheter for controlled removal of obstructions with laser energy must meet a number of important requirements. The catheter must be small in diameter and highly flexible so that it can be advanced to obstructions in small diameter vessels. It is frequently desired that the catheter be used with a guidewire which assists in positioning the catheter. In this case, the catheter must be specially adapted for use with a guidewire. The laser energy directed through the optical fiber or fibers must have sufficient intensity to remove the plaque deposits but must be controlled in a manner which eliminates the possibility of perforating the blood vessel wall with the laser energy. Vessel perforation may be prevented by controlling the laser energy pattern such that high intensity occurs only in a shallow working region just beyond the distal tip of the catheter.
It is usually desired that the laser energy form a hole in the obstruction of sufficient diameter to pass the catheter (approximately 1.5 mm). The catheter is advanced through the hole which it forms, and laser energy is repeatedly applied so as to form a passage through a long obstruction. It is customary to launch a laser beam into the proximal end of the optical fiber in a laser catheter with low divergence in order to provide good transmission through the fiber and to reduce bending losses in the fiber. However, the low divergence input laser beam results in the laser energy being emitted from the distal end of the optical fiber with a small angle of divergence, typically on the order of about 6.degree.. A small divergence laser beam passing through an optical fiber having a diameter on the order of 200 micrometers does not form a hole of sufficient diameter to pass the catheter without modification of the laser beam.
A hole approximately the diameter of the catheter can be obtained by expanding the laser beam which exits from the optical fiber with a lens arrangement and/or by utilizing multiple optical fibers. However, as the number of optical fibers is increased, the flexibility of the catheter decreases. Lens arrangements can, in theory, be utilized to expand the laser beam or beams. The aforementioned U.S. Pat. No. 4,817,601 discloses a laser catheter having a lens arrangement for providing a desired laser radiation pattern. However, in practice, lens assemblies of sufficiently small size to be used in a coronary angioplasty catheter are difficult to fabricate, particularly when multiple, small-diameter optical fibers are utilized. A lens assembly must be able to withstand the high laser energy levels required for removal of obstructions. Another approach to beam expansion is disclosed in the aforementioned U.S. Pat. No. 4,850,351. A window having sufficient thickness to permit the laser beams to diverge before emerging from the distal tip of the catheter is mounted at the distal end of the catheter. However, such a window reduces the flexibility of the tip region of the catheter.
Various other lens arrangements have been disclosed in the prior art. U.S. Pat. No. 4,800,876 issued Jan. 31, 1989 to Fox et al discloses a laser catheter for angioplasty including multiple optical fibers and a lens arrangement at the distal end of each optical fiber. A converging lens focuses the laser beam and prevents divergence toward the lumen wall, and a prism bends the laser beam toward the center of the lumen. U.S. Pat. No. 4,273,109 issued Jun. 16, 1981 to Enderby discloses an endoscope wherein light is transmitted through an optical fiber that is terminated in a lens system which can have either a converging or diverging characteristic. U.S. Pat. No. 4,266,534 issued May 12, 1981 to Ogawa discloses an illumination unit for an endoscope wherein a window covers the distal end of an optical fiber. The window includes spherical lens portions having different radii to provide a uniform illumination pattern. A collimator lens for an optical fiber including a fresnel lens pattern formed on a cylindrical member is disclosed in U.S. Pat. No. 4,815,807 issued Mar. 28, 1989 to Kaneko et al. An interlaced binary diffraction grating for laser beam profile shaping is disclosed by W. B. Veldkamp, Applied Optics, Vol. 21, No. 17, September 1982, pp. 3209-3212.
It is a general object of the present invention to provide improved catheters for controlled removal of biological obstructions with radiant energy.
It is another object of the present invention to provide a catheter having a diffraction grating at the distal end of an optical fiber to control the spatial distribution pattern of radiant energy emitted from the catheter.
It is a further object of the present invention to provide a catheter for removing biological obstructions with radiant energy, the radiant energy having a spatial distribution which reduces or eliminates the possibility of vessel wall perforation.
It is still another object of the present invention to provide a catheter which is small in diameter and highly flexible.
It is yet another object of the present invention to provide a catheter which is low in cost and easy to manufacture.
It is still another object of the present invention to provide a catheter having a working region wherein tissue is removed by radiant energy, the working region having a short axial depth and a diameter approximately equal to the diameter of the catheter.
It is a further object of the invention to provide an improved laser catheter for use in transluminal angioplasty.
It is another object of the present invention to provide a catheter including at least one optical fiber and means for modifying the spatial distribution pattern of laser radiation emitted from the optical fiber.